This invention relates to laser-induced damage to pigmented ocular cells.
Laser treatment of tissues within the eye is commonly used to treat diseases such as proliferative diabetic retinopathy, other forms of neovascularization, open-angle glaucoma, cataracts, and intraocular tumors. While laser treatment has greatly enhanced the performance of many types of ocular surgery, certain procedures are adversely affected by the nonselective nature of laser-induced tissue damage. Current ocular surgery lasers cause tissue coagulation or ablation, meaning total removal or vaporization, with no ability to discriminate between cell or tissue types.
For example, two types of ablative laser surgery are now commonly used to treat open-angle glaucoma. Glaucoma refers to abnormally elevated intraocular pressure, and is most commonly caused by increased resistance to outflow of the continuously produced aqueous humor. The aqueous humor exits the eye through the trabecular meshwork (TM) at the periphery of the anterior chamber. The TM consists of collagenous beams and plates, covered with phagocytic TM cells, and enclosing passages through which the aqueous humor flows, finally entering the vasculature via Schlemm's canal. Pathologic changes in the trabecular meshwork are thought to be involved in the etiology of certain forms of glaucoma.
The resistance by the TM is lowered, and intraocular pressure subsequently decreased, by laser surgery to ablate portions of the TM. Laser trabeculoplasty surgery utilizes long pulses (of approximately 100 msec) of laser radiation at high fluency, which is a measure of radiant exposure, or energy per area of tissue (mJ/cm.sup.2). This procedure causes immediate tissue coagulation and focal burns of the TM, as well as subsequent scarring in the burned areas. Aqueous flow is thought to be improved in the region adjacent to the lasered tissue. The trabeculoplasty procedure results in lowered intraocular pressure which lasts for only a limited time period. The scar tissue which forms after the procedure severely limits the efficacy of subsequent trabeculoplasties on a given patient. In an alternate procedure, trabeculopuncture, the absorption of laser radiation at high fluence and irradiance (Watts/cm.sup.2) causes photodisruption of the TM tissue, producing a hole through the TM into Schlemm's canal, the major outflow vessel. The laser-created trabeculopuncture channel into Schlemm's canal eventually fills in with fibrotic tissue; the beneficial effects of the procedure on intraocular pressure are not permanent.